Microlithography (also called photolithography or simply lithography) is a technology for the fabrication of integrated circuits, liquid crystal displays and other microstructured devices. The process of microlithography, in conjunction with the process of etching, is used to pattern features in thin film stacks that have been formed on a substrate, for example a silicon wafer. At each layer of the fabrication, the wafer is first coated with a photoresist which is a material that is sensitive to radiation, such as deep ultraviolet (DUV) light. Next, the wafer with the photoresist on top is exposed to projection light in a projection exposure apparatus. The apparatus projects a mask containing a pattern onto the photoresist so that the latter is only exposed at certain locations which are determined by the mask pattern. After the exposure the photoresist is developed to produce an image corresponding to the mask pattern. Then an etch process transfers the pattern into the thin film stacks on the wafer. Finally, the photoresist is removed. Repetition of this process with different masks results in a multi-layered microstructured component.
A projection exposure apparatus typically includes an illumination system for illuminating the mask, a mask stage for aligning the mask, a projection objective, and a wafer alignment stage for aligning the wafer coated with the photoresist. The illumination system illuminates a field on the mask that may have the shape of a rectangular or curved slit, for example.
In current projection exposure apparatus a distinction can be made between two different types of apparatus. In one type each target portion on the wafer is irradiated by exposing the entire mask pattern onto the target portion in one go. Such an apparatus is commonly referred to as a wafer stepper. In the other type of apparatus, which is commonly referred to as a step-and-scan apparatus or scanner, each target portion is irradiated by progressively scanning the mask pattern under the projection beam along a scan direction while synchronously moving the substrate parallel or anti-parallel to this direction. The ratio of the velocity of the wafer and the velocity of the mask is equal to the magnification of the projection objective, which is usually smaller than 1, for example 1:4.
It is to be understood that the term “mask” (or reticle) is to be interpreted broadly as a patterning mechanism. Commonly used masks contain transmissive or reflective patterns and may be of the binary, alternating phase-shift, attenuated phase-shift or various hybrid mask type, for example. However, there are also active masks, e.g. masks realized as a programmable mirror array. Also programmable LCD arrays may be used as active masks.
As the technology for manufacturing microstructured devices advances, there are ever increasing demands also on the illumination system. Ideally, the illumination system illuminates each point of the illuminated field on the mask with projection light having a well defined irradiance and angular distribution. The term angular distribution describes how the total light energy of a light bundle, which converges towards a particular point in the mask plane, is distributed among the various directions of the rays that constitute the light bundle.
The angular distribution of the projection light impinging on the mask is usually adapted to the kind of pattern to be projected onto the photoresist. For example, relatively large sized features may involve a different angular distribution than small sized features. The most commonly used angular distributions of projection light are referred to as conventional, annular, dipole and quadrupole illumination settings. These terms refer to the irradiance distribution in a system pupil surface of the illumination system. With an annular illumination setting, for example, only an annular region is illuminated in the system pupil surface. Thus there is only a small range of angles present in the angular distribution of the projection light, and thus all light rays impinge obliquely with similar angles onto the mask.
Different approaches are known in the art to modify the angular distribution of the projection light in the mask plane so as to achieve the desired illumination setting. In the simplest case a stop (diaphragm) including one or more apertures is positioned in a pupil surface of the illumination system. Since locations in a pupil surface translate into angles in a Fourier related field plane such as the mask plane, the size, shape and location of the aperture(s) in the system pupil surface determines the angular distributions in the mask plane. However, any change of the illumination setting involves a replacement of the stop. This makes it difficult to finally adjust the illumination setting, because this would involve a very large number of stops that have aperture(s) with slightly different sizes, shapes or locations.
Many common illumination systems therefore include adjustable elements that make it possible, at least to a certain extent, to continuously vary the illumination of the pupil surface. Conventionally, a zoom axicon system including a zoom objective and a pair of axicon elements are used for this purpose. An axicon element is a refractive lens that has a conical surface on one side and is usually plane on the opposite side. By providing a pair of such elements, one having a convex conical surface and the other a complementary concave conical surface, it is possible to radially shift light energy. The shift is a function of the distance between the axicon elements. The zoom objective makes it possible to alter the size of the illuminated area in the pupil surface.
For further increasing the flexibility in producing different angular distribution in the mask plane, it has been proposed to use mirror arrays that illuminate the pupil surface. In EP 1 262 836 A1 the mirror array is realized as a micro-electromechanical system (MEMS) including more than 1000 microscopic mirrors. Each of the mirrors can be tilted in two different planes perpendicular to each other. Thus radiation incident on such a mirror device can be reflected into (substantially) any desired direction of a hemisphere. A condenser lens arranged between the mirror array and the pupil surface translates the reflection angles produced by the mirrors into locations in the pupil surface. This known illumination system makes it possible to illuminate the pupil surface with a plurality of circular spots, wherein each spot is associated with one particular microscopic mirror and is freely movable across the pupil surface by tilting this mirror.
Illumination systems are also known from, for example, US 2006/0087634 A1, U.S. Pat. No. 7,061,582 B2 and WO 2005/026843 A2.
The geometry of the field illuminated in the mask plane is usually determined by a plurality of components. One of the most important components in this respect is an optical raster element which produces a plurality of secondary light sources in the system pupil plane. The angular distribution of the light bundles emitted by the secondary light sources is directly related to the geometry of the field illuminated in the mask plane. By suitably determining the optical properties of the optical raster element, for example the refractive power in orthogonal directions, it is possible to obtain the desired field geometry.
Usually it is desired that the geometry of the illuminated field can be varied at least to a certain extent. Since the optical properties of the optical raster element cannot be changed easily, a field stop is provided that is imaged by a field stop objective on the mask. The field stop usually includes a plurality of blades that can be individually moved so as to delimit the field illuminated in the mask. The field stop also ensures sharp edges of the illuminated field. In apparatus of the scanner type an adjustable field stop is used to open and shut the illuminated field at the beginning and the end of each scan process, respectively.
If the geometry of the illuminated field is varied with the help of an adjustable field stop, light losses are inevitable because a portion of the projection light is blocked by the blades of the field stop.